1. Field of the Invention
The present invention relates to a display device and a photoelectric conversion device with an image capturing function.
2. Related Art
A liquid crystal display device has an array substrate on which signal lines, scanning lines, and pixel thin film transistors (pixel TFTs) are arranged; and a driving circuit which drives the signal lines and scanning lines. A process technology by which a part of the driving circuit is formed on the array substrate has been put to practical use along with the recent development of integrated circuit technologies. Thereby, the whole size of the liquid crystal display device can be made thinner and smaller, and it has been widely used as display devices for various types of portable equipment such as a cellular telephone and a notebook computer.
Incidentally, display devices which have an image capturing function composed of contact type area sensors (photoelectric conversion devices) for image capturing on the array substrate have been proposed (refer to, for example, the Japanese Patent Application Publication No. 2001-292276, and the Japanese Patent Application Publication No. 2001-339640).
In a conventional display device with such an image capturing function, the amount of electric charge of the capacitors connected to the photoelectric devices is changed according to the amount of light received by the photoelectric conversion device, and image capturing is conducted by detecting a voltage at both ends of the capacitor.
As a technology which pixel TFTs and a driving are formed on the same glass substrate in a polysilicon process has been advanced in recent years, the photoelectric conversion device can be easily formed in each pixel, using the polysilicon process.
However, it is difficult to obtain an enough photoelectric current with the photoelectric conversion device using polysilicon. Though a technology by which a photoelectric conversion device is formed, using amorphous silicon, has been well known to date, the technology has a disadvantage that the manufacturing cost is increased, because a different amorphous silicon process from those of a pixel TFT and a driving circuit formed in the polysilicon process is required to be prepared.